The present invention enables the dew point or frost point of a gas to be measured under the condition of an instantaneous equilibrium between the solid and vapor phases of water, thereby assuring that the measured dew point or frost point corresponds to the saturation temperature of the gas while, at the same time, the vapor pressure of the gas corresponds to the saturation vapor pressure of ice at that temperature.
As has been pointed out in the references of many researchers, dictionaries and patents, it takes a very long time to measure dew points or frost points, particularly low dew points or frost points, and supercooling often takes place when the dew point or frost point to be measured is -100.degree. C. or below.
As a result of many repeated experiments, the present inventors have discovered a method that is believed to be the most accurate way of dew point or frost point measurement that is practical enough to be implemented with an industrially useful dew point or frost point meter.
It has been known to determine the water content of a gas by measuring its dew or frost point (see, for example, U.S. Pat. No. 5,052,818). In that method, the gas to be measured is blown against a reflecting mirror cooled to -80.degree. C. or below and the condensation of dew or frost on the reflecting mirror is detected by a sudden increase in scattered light and the water content of the gas is determined from the dew or frost point. However, later studies of the present inventors have shown that below -90.degree. C., the amount of condensation of dew or frost on the reflector mirror is so small that the detector sometimes fails to achieve the correct sensing of the point where such dew or frost condensation has occurred. Even if dew or frost condensation can be sensed by the detector, one often cannot be sure whether the sensed point of dew or frost condensation reflects the correct dew or frost point. In addition, if a phenomenon called "supercooling" occurs, dew or frost will not be condensed at the temperature where dew or frost condensation would otherwise occur. In this case, too, one if unable to know for sure whether the measured point reflects the correct dew or frost point.
As a result of the extensive studies they conducted, the present inventors found that when the reflector mirror on which dew or frost formed was further held at a lower temperature until the formation of a solid phase of water in a suitable amount and when heating and cooling steps were taken, preferably through at least one cycle, such that the temperature was controlled to decrease from the point where the intensity of scattered light was at a maximum or from the point where the intensity of reflected light was at a minimum to the point where the intensity of scattered or reflected light was at a specified level, the temperature at that point could safely be considered as the temperature of dew or frost formation. The present invention has been accomplished on the basis of this finding.